LIFE PROCESSES OF ANIMALS 365 



and echinoderms, and in some moUusks and worms these 

 cells emanate from two different individuals, male and 

 female. Both kinds of cells may be produced by the same 

 individual. For example, each earthworm produces both 

 the sperm cell and egg cell. Hydrse, sea anemones, jelly- 

 fish, and many worms are bisexual; that is, produce both the 

 sperm cell and egg cell. After the sperm cell unites with, 

 or fertilizes the egg cell, development of the embryo begins. 

 The process of growth has been described in Chapter III. 



